Abstract We examined conditions under which white shrimp (Litopenaeus setiferus) post-larvae enter an estuarine channel receiving high freshwater discharge and one receiving negligible discharge in the Ossabaw Sound system of Georgia, USA, during 1997 and 1998. We used surface nets to collect plankton over several 14-day periods, during which consecutive tows were made at night against the flooding current at stations in the inlet channels. During these sampling periods, additional intensive periods of around-the-clock surface and near-bottom (using a bottom sled) plankton tows were made. Data on oceanographic conditions were obtained from moored instrument arrays and shipboard sampling. We identified three key factors that influenced the densities of post-larval white shrimp in time and space within the Ossabaw inlet system. The first factor was a critical minimum temperature of coastal waters of 27–28°C. Once the threshold temperature was reached, lunar tidal stage became a key factor when the full duration of the flood tide coincided with darkness during peak ingress. This peak also coincided with an increase in water level within the system by more than 0.2 m, which induced an additional influx of water that reinforces the flood current over the ingress period. Our results suggest that the direction of subtidal currents (into or out of the system) becomes a significant factor in post-larval ingress when influx of water coincides with the time of favorable temperature conditions and nighttime flood tides.